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President Abraham Lincoln and his wife, Mary Todd, needed a baby-sitter.

It was April 18, 1864, and the Lincolns had planned to go to a fair in Baltimore, returning the next day. They needed someone to watch their 11-year-old son, Tad.

A newly published letter from Mrs. Lincoln requesting a sitter gives rare insight into the family's life inside the Lincoln White House, showing one way the family had to juggle their busy schedules, just like everyone else.

The letter is being offered for sale by the Raab Collection, a dealer in historical documents, valued at $15,000.

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It's not long -- just one line -- but the signed letter is in the first lady's hand.

"Hon. Mr. Harrington, We would like to have the services of Charles from today, at 2 P.M. until tomorrow at 11 A.M. Very Resp. Mrs. Lincoln."

"Charles" was Charles Forbes, a Treasury Department employee who was detailed to the Lincolns and often served as the president's valet, footman, messenger or attendant, according to the Raab Collection.

George Harrington was assistant secretary of the Treasury and Forbes' boss and handled personal financial matters for the Lincolns.

"Children in the White House have always held a great fascination with the American people," said Nathan Raab, vice president of the Raab Collection. "It shows the president and first lady at their most personal, their most human."

Mary Todd Lincoln ended up not going to the Baltimore event, likely too burdened with preparations for a reception the next night, the final White House reception of the season, Raab says.

Forbes' close family relationship soured a year later when President Lincoln was assassinated. Forbes was seated outside the president's box at Ford's Theatre and was the one who allowed the assassin, John Wilkes Booth, to enter. Forbes and the president's guard then left for a drink, leaving Lincoln unattended, Raab says.

Mary Todd Lincoln ended up blaming Forbes for her husband's death.

"Lincoln," a film about the 16th president and his battle to end slavery, is up for 12 Academy Awards this year, including best picture. The ceremony is scheduled to be broadcast Sunday night.